1. Field of the Invention
This invention applies to scanners of optical media which have protective covers.
2. Description of the Prior Art
A description of optical storage cards can be found in U.S. Pat. No. 4,360,728. These cards have a transparent plastic cover to protect the recording media.
Insensitivity to dust, dirt and fingerprints is increased by having a wider aperture lens which has a large diameter beam at the surface. On the other hand, lenses with a wide field of view required for a reasonable angle of scan do not have large apertures. For example, a lens used in a system where the media is moved under a small lens creating a single spot may have a numerical aperture of 0.6. A lens in a system with charge coupled device (CCD) or mirror scanner is unlikely to exceed a numerical aperture of 0.25. The numerical aperture is the ratio of the lens diameter to focal length.
A solution which can support wide angle scanning and produces a large beam at the protective cover surface is described It is based on cyclindrical lenses.
A prior art system for reading optical storage cards is shown in FIG. 1. The microscope objective enlarges a strip of around 1-1.5 mm. on the card to an image of about 1/4 inch on the surface of the CCD and has a numerical aperture around 0.25.
Another prior art system is shown in FIGS. 2A, 2B and 2C, which shows a mirror scanner which could scan a 16 mm. strip with a laser beam. Its numerical aperture will be no better than the system of FIG. 1. In fact, available lenses will have a smaller aperture because they have a wider field of view. In FIGS. 2A, 2B and 2C, the various elements are as follows: Element 1 is a fixed mirror approximately 32 mm. by 7 mm., 2 is a telecentric lens, 3 is a mirror approximately 6 mm. by 12 mm. rotated by a galvanometer (not shown), 4 is a fixed mirror, 5 is a quarter wave retarder, 6 is a microscope objective, 7 is a collimator and light source--probably a laser, 8 is a polarization splitter, 9 is a detector, 10 is the optical storage card, and 11 is a stigmatic lens element used if focus error signal is required (this is a section of ophthalmic lens used to correct an eye for astigmatism). In this system, for a smaller spot, change the collimator and lens parameters and increase the aperture of system. Note that the optics could be swapped around to bring the card nearer the center. The mirror angle effective "lever arm" is 25 mm., the plus or minus scan is 8 mm. and the mirror rotation is 17 degress giving a total scan of 34 degrees.